1. Field of the Invention
This invention relates to color printing having a plural highlight color image mapped from a full color image. More particularly, this invention is directed to multiple highlight color printing which preserves information important to the viewer.
2. Description of the Related Art
Color images are a significant element in today's printing industry. As a result, electronic color printers and color image creation tools have been increasingly developed to obtain color images using electronic printing methods. Color printing is often performed using full color. The gamut of colors includes tints and shades of a full color spectrum--reds, greens, blues and their combinations.
A significant amount of color printing, however, can be performed using highlight color printing rather than full color printing. In highlight color printing, when a single highlight color is used, only two inks are used in the printing process. These inks are black and a highlight color, which is often red or blue. Electronic printers may be designed specifically for highlight color printing. A highlight color printer is generally faster and less expensive than a full color printer, as only two inks are processed, rather than the three or four inks which must be processed in order to obtain full color images.
The gamut of full colors is a three-dimensional volume which can be represented by the double hexagonal cone 10 illustrated in FIG. 1. In this representation, shades vary from dark to light as one moves upwards from the black vertex. Tints vary from unsaturated grays to fully saturated colors as one moves out radially from a central axis. Hues vary as one moves angularly in a horizontal plane.
The gamut of colors available to a single highlight color printer can be represented by the two-dimensional triangle 13 illustrated in FIG. 2. This is a slice from the full color double-hexagonal cone of FIG. 1 at the angle of the highlighted hue.
Prior attempts to print a full color image on a highlight color image involve mapping the three-dimensional double hexagonal cone of FIG. 1 to a two-dimensional triangular slice of colors within the double-hexagonal cone. The highlight color printer attempts to render the highlight color image by mapping the full color specification onto the set of colors which it can produce. In this mapping, many different colors in the full color space are mapped to the same color in the highlight color space. This information important to the viewer is often lost.
U.S. Pat. No. 4,554,241 to Edwards discloses a method of printing a realistic image of an original on a sheet. Two printing plates are used to print different impressions with two different coloring media.
U.S. Pat. No. 4,636,839 to Cole et al. discloses a method and an apparatus for generating color matte signals. RGB color components are calculated from hue, saturation and luminance value inputs. A new luminance value Y and color difference components CR and CB are calculated from these RGB values. Various algorithms are disclosed which are used for color calculation.
U.S. Pat. No. 4,670,780 to McManus et al. discloses a method for matching hard copy colors to display colors for registered ink jet copiers. A color transformation for matching hard copy color to display color includes an MSW color space which restricts the hard copy color to certain percentages of binary mixtures of inks (M), single inks (S) and paper white (W). Color data in the XYZ space is converted to the MSW space. Unreachable display colors are matched into reachable hard copy colors. Corrections for color shifts due to interactions of the inks are also disclosed.
U.S. Pat. No. 4,761,669 to Langdon discloses an electrophotographic highlight color printing machine in which printing is done in at least two different colors. Methods for transferring multiple color images simultaneously are disclosed.
U.S. Pat. No. 4,682,186 to Sasaki et al. discloses a method for forming a color image. The color image is formed by using a plurality of coloring materials and controlling quantities of the coloring materials. When a density to be reproduced exceeds the density reproducible for that coloring material, the quantities of other coloring materials are reduced.
U.S. Pat. No. 4,907,078 to Hasebe discloses a method of reproducing color images. Two charge-coupled devices of different wavelengths are used to scan a full color document. The outputs of the charge-coupled devices are fed into a lookup table to determine the appropriate color in a two-color system. The output from the table is then recorded on a piece of paper. The system can be used for any type of copier.
U.S. Pat. No. 4,894,665 to Davis discloses a method of generating an expanded color set of a low resolution color printer. The four-color printer can be expanded to twelve colors by printing a black dot next to a line to make the line seem darker. Two algorithms are provided for determining whether or not a line is critical and for enhancing a line.
U.S. Pat. No. 4,908,779 to Iwata discloses a display pattern processing apparatus. The system can be programmed to convert a full color image into a number of other formats. An example is shown where an RGB image is converted into a two-color image.
U.S. Pat. No. 5,237,517 to Harrington et al. discloses a method and an apparatus for mapping from a full color specification to a highlight color. The full color image is sent to a highlight printer using only a single highlight color. Harrington shows that mapping to a restricted set of colors entailed a loss of information.
The article entitled, "Reproducing Color Images as Duotones", Joanne Power et al., 1996 SIGGRAPH PROCEEDINGS, pp. 237-248, discloses a method of mapping a full color image to a two-color print. The reference describes utilizing an orthogonal system to define the printable color gamut and then transforms the image colors along two directions and uses parallel projections along the third. The method provides one of the orthogonal directions corresponding to the Y-axis of the XYZ color space and initially transforms in this direction. This method tends to have the effect of reducing the available black values, and therein negatively affects pictorial images.
While these references attempted to map a full color image to a highlight color image, they did not fully account for the need for mapping due to different image types. These different image types include pictorial images and presentation graphic images. These image types typically encode their useful information in different manners.